Catalysis of Organic..

120TEMPO Oxidation of Alcoholsthe use of hypohalites very frequently leads to the formation of undesirablehalogenated by-products thus necessitating further purification of the oxidationproduct. Extensive review of TEMPO based oxidation methods can be found in theliterature (10,11,12) and the industrial application of a highly efficient solvent- andbromide-free system has been reported (13,14).In recent years, much effort has been spent on developing both selective andenvironmentally friendly oxidation methods using either air or oxygen as theultimate, oxidant. One of the most selective and efficient catalyst systems reported todate is based on the use of stable nitroxyl radicals as catalysts and transition metalsalts as co-catalysts (15). The most commonly used co-catalysts are (NH 4 ) 2 Ce(NO 3 ) 6(16), CuBr 2 -2,2’-bipiridine complex (17), RuCl 2 (PPh 3 ) 3 (18,19), Mn(NO 3 ) 2 -Co(NO 3 ) 2 and Mn(NO 3 ) 2 -Cu(NO 3 ) 2 (20). However, from an economic andenvironmental point of view, these oxidation methods suffer from one commondrawback. They depend on substantial amounts of expensive and/or toxic transitionmetal complexes and some of them require the use of halogenated solvents likedichloromethane, which makes them unsuitable for industrial scale production.XScheme 1.3 N . O(1mol%)HH H+ O+ H 2 O2OOHNBS (1mol%)1 2Mg(NO 3 ) 2 , (1mol%)40-50 o C, 40-50psi, AcOHX = H, TEMPO (3); X = OCH 3 , MeO-TEMPO (4); X = NHCOCH 3 , AATEMPO (5)Very recently, Hu et al. claimed to have discovered a convenient procedure forthe aerobic oxidation of primary and secondary alcohols utilizing a TEMPO basedcatalyst system free of any transition metal co-catalyst (21). These authors employeda mixture of TEMPO (1 mol%), sodium nitrite (4-8 mol%) and bromine (4 mol%) asan active catalyst system. The oxidation took place at temperatures between 80-100 °C and at air pressure of 4 bars. However, this process was only successful withactivated alcohols. With benzyl alcohol, quantitative conversion to benzaldehydewas achieved after a 1-2 hour reaction. With non-activated aliphatic alcohols (suchas 1-octanol) or cyclic alcohols (cyclohexanol), the air pressure needed to be raisedto 9 bar and a 4-5 hour of reaction was necessary to reach complete conversion.Unfortunately, this new oxidation procedure also depends on the use ofdichloromethane as a solvent. In addition, the elemental bromine used as a cocatalystis rather difficult to handle on a technical scale because of its high vaporpressure, toxicity and severe corrosion problems. Other disadvantages of this systemare the rather low substrate concentration in the solvent and the observed formationof bromination by-products.